Structure and method for bond pads of copper-metallized integrated circuits

ABSTRACT

A metal structure for a contact pad of a wafer or substrate ( 101 ), which have copper interconnecting traces ( 102 ) surrounded by a barrier metal layer ( 103 ). The wafer or substrate is protected by an insulating overcoat ( 104 ). In the structure, the barrier metal layer is selectively exposed by a window ( 110 ) in the insulating overcoat. A layer of copper ( 105 ), adherent to the barrier metal, conformally covers the exposed barrier metal. Preferably, the copper layer is deposited by sputtering using a shadow mask. A layer of nickel ( 106 ) is adherent to the copper layer and a layer of noble metal ( 106 ) is adherent to the nickel layer. The noble metal may be palladium, or gold, or a palladium layer with an outermost gold layer. Preferably, the nickel and noble metal layers are deposited by electroless plating.

FIELD OF THE INVENTION

The present invention is related in general to the field ofmetallurgical systems with application to electronic systems andsemiconductor devices and more specifically to structure and methods forintegrated circuit contact pads bondable by ball bonding techniques.

DESCRIPTION OF THE RELATED ART

It is a continuing trend in the semiconductor industry to miniaturizeintegrated circuits (ICs). As a consequence of this trend, the RC timeconstant of the interconnection between active circuit elementsincreasingly dominates the achievable IC speed-power product.Consequently, there is a strong need to replace the relatively highimpedance of the interconnecting aluminum metallization by the lowerimpedance of metals such as copper.

For IC bond pads made of copper, the formation of thin copper(I)oxidefilms during the manufacturing process flow severely inhibits reliableattachment of bonding wires, especially for conventional gold-wire ballbonding. In contrast to aluminum oxide films overlying metallicaluminum, copper oxide films overlying metallic copper cannot easily bebroken by a combination of thermo-compression and ultrasonic energyapplied in the bonding process. As further difficulty, bare copper bondpads are susceptible to corrosion.

In order to overcome these problems, the industry favors a process, inwhich a layer of aluminum is formed as a cap over the copper bond pad;this process reconstructs in principle the traditional situation of analuminum pad, for which the conventional gold-wire ball bonding is wellcontrolled. This process, though, has a number of drawbacks such asadded cost and the risk of inadvertent scratching or smearing of thealuminum, causing electrical shorts.

Alternative processes based on the concept of depositing one or morelayers on the copper, which are reliably bondable, have until now raninto technical problems, such as insufficient adhesion among the variousmetal layers and insulating materials, or unexpected corrosion andchemical undercuts.

SUMMARY OF THE INVENTION

Applicants recognize a need for a straightforward solution to create ametallurgical bond pad structure suitable for ICs having copperinterconnection metallization, which combines a low-cost method offabricating the bond pad structure with high reliability in operatingthe structure, in particular with reduced possibility of delaminationand corrosion. It is a technical advantage that the bond pad structureand the method of fabrication are flexible enough to be applied fordifferent IC product families and a wide spectrum of design and processvariations. Preferably, these innovations should be accomplished whileshortening production cycle time and increasing throughput and yield,and without the need of expensive additional manufacturing equipment.

One embodiment of the invention is a metal structure for a contact padof a substrate having copper interconnecting traces surrounded by abarrier metal layer, wherein the substrate is protected by an insulatingovercoat. In the structure, the barrier metal layer is selectivelyexposed by a window in the insulating overcoat. A layer of copper,adherent to the barrier metal, conformally covers the exposed barriermetal. A layer of nickel is adherent to the copper layer and a layer ofnoble metal is adherent to the nickel layer. The noble metal may bepalladium, or gold, or a palladium layer followed by a gold layer.

Another embodiment of the invention is a method for fabricating a metalstructure for a contact pad of a substrate having copper interconnectingtraces surrounded by a barrier metal layer, wherein the substrate isprotected by an insulating overcoat. First, a window is opened in theinsulating overcoat to selectively expose the barrier metal layer. Ashadow mask is then provided, which has an opening matching the contoursof the window; the mask has a weak adhesive on one surface. The mask isaligned with the substrate so that said the opening is aligned with theovercoat window. The adhesive mask surface is brought in contact withthe overcoat. A layer of copper is then deposited on the barrier metallayer, preferably by a sputtering technique; thereafter, the shadow maskis removed. A layer of nickel is deposited on the copper layer,preferably by an electroless plating technique. Finally, a layer ofnoble metal is deposited on the nickel layer, preferably by electrolessplating.

Embodiments of the present invention are related to wire-bonded ICassemblies, semiconductor device packages, surface mount and chip-scalepackages. It is a technical advantage that the invention offers alow-cost method of sealing the bond pad against moisture anddelamination, and thus for protecting the integrated circuit againstcorrosion and stress-related contact failure. It is an additionaltechnical advantage that the invention offers a methodology to smooth,stable interfaces of the gold wire ball and the modified bond pad,resulting in welds with strong metallic interdiffusion. Furthertechnical advantages include the opportunity to scale the assembly tosmaller dimensions, supporting the ongoing trend of IC miniaturization;and the absence of unwanted metals, supporting high reliability of thefinished IC assemblies.

The technical advantages represented by certain embodiments of theinvention will become apparent from the following description of thepreferred embodiments of the invention, when considered in conjunctionwith the accompanying drawings and the novel features set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of a bonded contact pad illustratinga preferred embodiment of the invention.

FIGS. 2 to 6B are schematic cross sections illustrating certain steps ofthe process flow for completing the fabrication of a bond pad on anintegrated circuit wafer.

FIG. 2 depicts the opening of a window in the insulating overcoat toexpose a portion of the interconnecting traces.

FIG. 3 depicts the alignment and attachment of a shadow mask matchingthe contours of the window.

FIG. 4A depicts the plasma deposition of a copper layer on the exposedtrace.

FIG. 4B depicts the plasma depositions of a refractory metal layer and acopper layer on the exposed trace.

FIG. 5A depicts the deposition of a nickel layer on the copper layer ofFIG. 4A.

FIG. 5B depicts the deposition of a nickel layer on the copper layer ofFIG. 4B.

FIG. 6A is the deposition of a noble metal layer on the nickel layer ofFIG. 5A.

FIG. 6B is the deposition of a noble metal layer on the nickel layer ofFIG. 5B.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The schematic cross section of FIG. 1 illustrates a bond pad generallydesignated 100 of a semiconductor device, completed according to theprocess flow of the invention and with a wire bond attached. A substrate101 has interconnecting traces 102 made of copper. Substrate 101 may bean integrated circuit formed in a semiconductor material, or it may be asupporting board. For an integrated circuit, copper trace 102 haspreferably a thickness in the range from about 0.2 to 1.0 μm. In orderto prevent outdiffusion of the copper, trace 102 is preferablysurrounded by a barrier metal layer 103. Barrier metal layer 103 istypically tantalum nitride in the thickness range from about 20 to 50nm. Substrate 101 is protected by an insulating overcoat 104, typicallymade of silicon nitride, silicon oxynitride, silicon carbide, or stacksof these or related insulators; a preferred thickness range is fromabout 0.5 to 1.0 μm.

As FIG. 1 shows, the barrier metal layer 103 of the copper trace 102 isselectively exposed by the window 110 in the insulting overcoat 104.Adherent to the exposed barrier metal layer 103 is a layer 105 ofcopper, which is preferably in the thickness range from about 0.2 to 0.5μm. Adherent to copper layer 105 is a layer 106 of nickel, which ispreferably in the thickness range from about 0.2 to 0.5 μm. Adherent tonickel layer 106 is a layer 107 of noble metal. Preferably this noblemetal is palladium in the thickness range from about 100 to 300 nm.Alternatively, it may be gold about 50 to 120 nm thick, or it may be astack of a palladium layer with an outermost layer of gold.

Layer 107 of noble metal is bondable by wire ball bonding. FIG. 1 showsa typical ball 120 formed of wire 121 and welded to noble metal layer107, preferably by an automated and commercially available wire bonder.

Other embodiments of the invention address different bond padconfigurations. As an example, copper trace 102 may have the barriermetal layer 103 removed across the width of window opening 110, afterthe window has been opened. In such device, a layer of refractory metalis deposited onto the copper of the interconnecting trace so that itadheres to the copper trace. Examples of refractory metals are titanium,tungsten, chromium, molybdenum, or alloys thereof; other examplesinclude stacks of these metals or alloys. The preferred thickness rangeis from about 0.1 to 0.3 μm. In some embodiments, a refractory metallayer may even be deposited onto the existing barrier metal layer as anadditional adherent layer.

Another embodiment of the invention is a method for completing a metalstructure for a contact pad of a substrate, which has interconnectingtraces of copper surrounded by a barrier metal layer. Certain processsteps are illustrated in the schematic cross sections of FIGS. 2 to 6B.In FIG. 2, the substrate is designated 201 and the copper trace 202. Abarrier metal layer 203 surrounds copper trace 202. A preferred barriermetal is tantalum nitride. An insulating overcoat 204 protects thesurface of substrate 201 and metal trace 202. A preferred overcoatmaterial is silicon nitride or silicon oxynitride.

The process flow starts by opening a window of width 210 in overcoat 204to expose a portion of the metallization trace. In the device of FIG. 2,the metal actually exposed is the barrier metal (for example, tantalumnitride); in other devices, it is the copper of the trace. In the nextprocess step, depicted in FIG. 3, a shadow mask 301 is provided, whichhas an opening 310 matching the contours 210 of the window in theovercoat. Mask 301 has an adhesive 302 on one surface 301 a; adhesive302 is weak so that mask 301 adheres safely to overcoat 204 during theprocess steps in the vacuum chamber, but after the vacuum operation, theadhesive offers little resistance to peeling off mask 301 from overcoat204. Shadow masks are stamped from metal foil (typically aluminum), areinexpensive and may be disposable or re-usable.

Shadow mask 301 is aligned with the overcoat window so that the maskopening 310 is aligned with the overcoat window 210. The weak adhesive302 on mask surface 301 a is then brought in contact with the overcoat204. The resulting arrangement, illustrated in FIG. 3, is transferredinto a vacuum chamber for sputter depositing metal layers.

FIGS. 4A and 4B indicate plasma processes 410 in the vacuum chamber.First, the surface of the exposed trace may be cleaned by subjected itfor a short period of time to a plasma of inert gases such as argon ornitrogen. Without breaking the vacuum of the chamber, the sputterdeposition of refractory metal layer 401 is then initiated, as indicatedin FIG. 4A (the simultaneously deposited refractory metal on top of mask301 is not shown in FIG. 4A). Refractory metal layer 401 has a preferredthickness range from about 0.1 to 0.3 μm. It may be selected from agroup of refractory metals and alloys; a preferred selection is an alloyof titanium and tungsten in a mixture of about equal amounts.Alternative choices include chromium, molybdenum, alloys of these andother refractory metals, or stacks of refractory metal layers. Layer 401is required, when the barrier metal layer 203 is missing on the exposedsurface of copper trance 202; layer 401 may be omitted, when barrierlayer is present on the surface of copper trace 202, as shown in FIG.4B.

Without breaking the vacuum, a layer 402 of copper is then sputtered onthe refractory metal 401 (FIG. 4A), or on the barrier metal 203 (FIG.4B). The copper is adherent to the metal layer underneath. The preferredthickness range for the copper layer is between about 0.2 to 0.5 μm.

After the sputter deposition of the copper layer, the wafer is takenfrom the vacuum chamber and shadow mask 301 is removed from the wafer.The mask may be used only one time and then disposed, or it may becleaned for re-use. In addition, the wafer surface may be cleaned toremove any residual adhesive.

The deposition of the additional metal layers for completing the bondpad structure is performed using an electroless plating technique. InFIGS. 5A and 5B, a nickel layer 501 is (electroless) plated on thepristine copper layer 402. Nickel layer 501 is adherent to the pristinecopper layer 402 and is preferably between about 0.2 and 0.5 μm thick.

Finally, a layer 601 of noble metal is (electroless) plated on nickellayer 501. Preferably, noble metal layer is made of palladium in thethickness range from about 100 to 300 nm. In many embodiments, though,an additional gold layer in the thickness range from about 50 to 120 nmis (electroless) plated on top of the palladium layer to form theoutermost layer of the structure. In either case, the noble metal layer601 adheres well to the underlying nickel layer 501.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the embodiments are effective insemiconductor wafers as well as in substrates, which have copperinterconnecting metallization yet need to be bondable using conventionalball or ribbon bonding. As another example, the material of thesemiconductor material may comprise silicon, silicon germanium, galliumarsenide, or any other semiconductor or compound material used in ICmanufacturing. As yet another example, the concept of the invention iseffective for many semiconductor device technology nodes and notrestricted to a particular one. It is therefore intended that theappended claims encompass any such modifications or embodiments.

1. A metal structure for a contact pad of a substrate having copperinterconnecting traces surrounded by a barrier metal layer, saidsubstrate protected by an insulating overcoat, comprising: said barriermetal layer selectively exposed by a window in said insulating overcoat;a layer of copper, adherent to said barrier metal layer, conformallycovering said exposed barrier metal; a layer of nickel adherent to saidcopper layer; and a layer of noble metal adherent to said nickel layer.2. The structure according to claim 1 wherein said substrate is anintegrated circuit formed in semiconductor material.
 3. The structureaccording to claim 1 further comprising a layer of refractory metaladherent to said exposed barrier metal layer, said refractory metallayer positioned between said barrier metal layer and said copper layer.4. The structure according to claim 1 wherein said noble metal ispalladium.
 5. The structure according to claim 1 wherein said noblemetal is gold.
 6. The structure according to claim 1 wherein said noblemetal is a palladium layer adherent to said nickel layer and anoutermost gold layer adherent to said palladium layer.
 7. The structureaccording to claim 1 wherein said barrier metal layer is tantalumnitride in the thickness range from about 20 to 50 nm.
 8. The structureaccording to claim 1 wherein said copper traces have a thickness in therange from about 0.2 to 1.0 μm.
 9. The structure according to claim 1wherein said insulating overcoat comprises silicon nitride, siliconoxynitride, silicon carbide, or stacks of these or related compounds inthe thickness range from about 0.5 to 1.5 μm.
 10. The structureaccording to claim 1 wherein said refractory metal is selected from agroup consisting of titanium, tungsten, chromium, molybdenum, or alloysthereof, or stacks thereof, in a thickness range from about 0.1 to 0.3μm.
 11. The structure according to claim 1 wherein said copper layer hasa thickness from about 0.2 to 0.5 μm.
 12. The structure according toclaim 1 wherein said nickel layer has a thickness from about 0.2 to 0.5μm.
 13. The structure according to claim 1 wherein said palladium layerhas a thickness from about 100 to 300 nm.
 14. The structure according toclaim 1 wherein said gold layer has a thickness from about 50 to 120 nm.15. A method for completing a metal structure for a contact pad of asubstrate having interconnecting traces of copper surrounded by abarrier metal layer, said substrate protected by an insulating overcoat,comprising the steps of: opening a window in said insulating overcoat toexpose a portion of said interconnecting trace; providing a shadow maskhaving an opening matching the contours of said window, said mask havinga layer of adhesive material on one surface; aligning said mask withsaid substrate so that said mask opening is aligned with said overcoatwindow; bringing said adhesive mask surface in contact with saidovercoat; depositing a layer of copper on said exposed portion of saidinterconnecting trace; removing said shadow mask; depositing a layer ofnickel on said copper layer; and depositing a layer of noble metal onsaid nickel layer.
 16. The method according to claim 15 wherein saidshadow mask is a stamped metal mask selected for single-time usage. 17.The method according to claim 15 further comprising the step ofdepositing a layer of refractory metal on said exposed portion of saidinterconnecting trace, followed by said step of depositing a layer ofcopper.
 18. The method according to claim 15 wherein said step ofdepositing said layer of copper comprises a sputtering technique. 19.The method according to claim 17 wherein said step of depositing saidlayer of refractory metal comprises a sputtering technique.
 20. Themethod according to claim 15 wherein said steps of depositing said layerof nickel and said layer of noble metal comprise an electroless platingtechnique.
 21. The method according to claim 15 further comprising thestep of cleaning said overcoat surface to remove any residual adhesiveafter said step of removing said shadow mask.
 22. The method accordingto claim 15 wherein said noble metal is palladium.
 23. The methodaccording to claim 15 wherein said noble metal is gold.
 24. The methodaccording to claim 15 wherein said step of depositing a noble metalcomprises first a step of depositing a layer of palladium on said nickellayer and then a step of depositing a layer of gold on said palladiumlayer.
 25. A method for completing a metal structure for a contact padof a substrate having interconnecting traces of copper surrounded by abarrier metal layer, said substrate protected by an insulating overcoat,comprising the steps of: opening a window in said insulating overcoat toexpose a portion of said interconnecting trace; providing a shadow maskhaving an opening matching the contours of said window, said mask havinga layer of adhesive material on one surface; aligning said mask withsaid substrate so that said mask opening is aligned with said overcoatwindow; bringing said adhesive mask surface in contact with saidovercoat; sputter-depositing a layer of refractory metal on said exposedportion of said interconnecting trace; sputter-depositing a layer ofcopper on said refractory metal layer; and removing said shadow mask.26. The method according to claim 25 further comprising the step ofdepositing at least one layer of bondable metal on said pristine copperlayer.